[MuffinFlavored]

is this separate to pip's "requirements.txt" usually?

[kortex]

Very different. Requirements.txt only handles dependency packages, and only handles on kind of dependency (no dev/release distinction). pyproject.toml does dev/release deps, it has package metadata, it can configure all your lint/test tooling, and it is extensible with plugins.

[MuffinFlavored]

and they can't be combined, right? like package.json for example

[senand]

Yes, it's assuming poetry is used, whereby normally the packages needed by the project are defined in pyproject.toml